I have a wishlisted book that is in good condition, except the cover has been bent and has a crease in the middle of it. Would that make it unpostable? I looked on the posting guidelines and it seems that it would be fine, but I want to make sure before I post it.

However, that doesn't mean people won't get upset about a creased cover. If you are concerned about it, you can always try posting it in one of the WL threads in the genre forums. At least there you have the opportunity to mention condition, and the book will go to someone who is just fine with a bit of creasing.